Amp Mods · Blues Jr. Reverb Footswitch Mod Kit
Fender Blues Jr Reverb Footswitch Mod Kit
Thanks for purchasing the Blues Jr. Reverb Footswitch Mod Kit. This mod will convert the footswitch on the back of your Blues Jr. amplifier controlling the on/off of the FAT circuit into a footswitch jack that will control the on/off of your reverb.
Optional Tools/Supplies:
- Silicone glue.
- 9/16” nut driver or socket wrench.
- 1/2” nut driver or socket wrench.
- Fine point Sharpie or similar permanent marker.
- Masking Tape
Skills required:
- Basic soldering skills .
- Basic knowledge of electronics.
Parts
- Latching SPST Footswitch
- 12” Wire
- 5W Power Resistor
Tools
- Temperature Controlled Soldering Iron and Solder
- Solder Sucker and or de-soldering braid
- Multi-Meter Capable of reading 500V DC
- Phillips Screwdriver
- Pliers with insulated handle
- Wire Cutters
- Safety Goggles
- Alligator Leads
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Step 1
- WARNING: Performing these modifications WILL void the warranty on your amplifier. Working on amps can be deadly if you do not take proper precautions. Follow these instructions very carefully. If you are at all unsure of your ability to perform these modifications STOP and take your amp to a professional to be modified. You can also send your amp to me for modification.
- No Warranty: This kit and the instructions included do not include any warranty of any kind. The modifications to your amplifier are at your own risk and you agree to completely hold harmless the seller of this kit against any and all claims. Tube amplifiers contain parts that operate at very high temperatures and deadly voltages. If you are not sure of your abilities to perform these modifications then do not perform them.
2
Prep the Amp
- Instructions: Follow these instructions to the letter.
- It’s a good idea to have all the parts and tools you will need laid out before starting in a well lit area.
- Unplug the Amp and put on your safety goggles.
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Step 3
- Remove the chicken head control knobs and the
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Step 4
- Remove the 6 screws holding on the back panel and remove the panel. If the back panel does not come off easily you can loosen the two chassis screws on the side of the amp about a half turn each.
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Step 5
- Drain the Filter Caps: The filter caps on your amp may be drained already but you must follow these steps to insure that any deadly voltage is removed from the amp before you begin work. Test the voltage across the filter caps. Set your multi meter to DC and measure the voltage across the positive and negative side of any of the filter caps. If the voltage reads higher than 10 volts, then you need to drain the caps.
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Step 6
- Using the insulated pliers to hold the power resistor touch one lead of the included 5W power resistor to the negative side of the filter cap and the other to the positive side, leave the leads in contact with the power resistor for 30 seconds and measure again. Keep repeating this step until the voltage reads below 10V.
- Capacitors can recharge even when the amp is not plugged in!!!
- After the filter caps are drained take an alligator lead and clip one end to the positive lead of the filter cap and the other end to the chassis and leave it there until you are done with your work. This will prevent the filter caps from re-charging.
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Step 7
- Remove the nuts holding the jack for the speaker and FAT switch with a ½” nut driver or socket wrench. Note: The lock nuts for these jacks are inside the chassis between the jack and chassis, not between the nut and the chassis.
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Step 8
- Unplug all the power and output transformer leads by pulling firmly with steady pressure gently rocking the lead terminal from side to side. As you remove each lead mark the wire with masking tape and a Sharpie so you will know where to plug it back in when you are putting the amp back together. I just mark the terminal number on the lead.
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Step 9
- Remove all seven (7) circuit board screws.
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Expose the circuit board
- Pull gently on the board to make sure that it is not stuck to any of the standoffs that were holding the circuit board screws. Move the small board holding the jacks for the speaker and FAT switch out of the way, hanging over the edge of the chassis.
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Step 11
- Press the wires from the output transformer and reverb as far away from the board and as close to the chassis as possible. The board needs to come down toward the bottom of the amp before the pots will clear the top of the chassis as shown.
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Step 12
- Using gentle pressure on the top of the circuit board press the board down making sure it is not getting hung up on any wires until all the pots have cleared the top of the chassis then pull the board to you to expose the underside. DO NOT PUT PRESSURE ON ANY COMPONENTS INCLUDING THE FILTER CAPS WHEN MOVING THE BOARD.
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For MADE IN MEXICO Blues Jr.’s perform the following
- Clip the red wire at WJ14 and the white wire at WJ15. Leave about an inch of white wire to bridge WJ15 and WJ14 as shown below.
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Step 14
- Bridge WJ15 and WJ14 as shown below using the excess wire left at WJ15. This will restore the FAT switch at the control panel
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Step 15
- Move the red wire that was clipped from WJ14 to the top side of R42 and solder it.
- Your footswitch jack will now ground the reverb circuit when engaged!
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For MADE IN USA Blues Jr.’s perform the following
- Clip the red wire at P11 and the white wire at P9, leaving about and inch and a half of white wire on P9.
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Step 17
- Bridge P9 to P11 as shown. This will restore the FAT switch at the control panel.
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Step 18
- Clip to remove the white wire at P15 on the footswitch/speaker jack PCB.
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Step 19
- Remove the red wire at P16 and install the included 12inch wire at P16.
- New wire installed at P16
- Removed White wire at P15
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Step 20
- Run the newly installed wire from P16 at the footswitch/speaker jack PCB to the right side of R48 as shown and solder it.
- Your footswitch jack will now ground the reverb circuit when engaged!
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Putting the Amp Back Together
- Make sure that you have clipped all the excess leads from the solder side of the board and that all your new solder joints are secure before continuing.
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Step 22
- Carefully put the PCB back into place making sure that the pots, fat switch, and input jack all line up with the control panel.
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Step 23
- Replace all 7 of the PCB mounting screws.
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Step 24
- Re-install the LED
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Step 25
- Make sure all the ribbon cables have a smooth path with no kinks or sharp turns in the cable.
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Step 26
- Dress the Transformer leads!!! The poor lead dress is a large part of the noise in this amp. Instead of using a new cable tie to put the wires back like we found them we are going to use standard wiring techniques for proper tube amp design. Twist the green heater wires and position as shown. All other lead pairs should cross each other at 90 degree angles to avoid noise, hum, and oscillation.
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Step 27
- Re-install the small board with the Speaker and FAT switch jack, remember that the lock washer goes on the inside of the chassis between the chassis and the jack. Do not tighten the nut too much on the jacks or you will strip out the jacks.
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Step 28
- Make sure all the ribbon cables have a smooth path with no kinks or sharp turns in the cable.
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Step 29
- Glue ALL the caps to the board as well as the lead wire for the reverb. You can omit this step if you want but securing all the caps to the board will decrease the likely hood that they will get shaken loose in travel or by vibrations from the speaker.
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Step 30
- Use cable ties as necessary to make the cables nice as tidy.
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Step 31
- Plug the speaker back into the speaker jack.
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Step 32
- Re-install the control knobs and install the washer and nut on the new input jack.